The present invention relates to the control of flowering time in plants using genetic engineering. Specifically, this invention relates to the control of flowering time in plants by manipulation of the activity of the FPA gene.
In growing plantlets, the transition from vegetative growth to flowering is the major developmental switch in the plant life cycle. The timing of this transition to a flowering state is critical for the plant's reproductive success. Accordingly, most plant species have evolved systems to precisely regulate flowering time. These systems monitor both environmental cues and the developmental state of the plant.
Photoperiod and temperature are two environmental cues commonly monitored by plants. In plants responsive to photoperiod cues so examined, flowering is promoted by flowering signals which are translocated from leaves to meristems as leaves detect day length changes (Zeevaart, Light and the Flowering Process, 137–142 (Eds., D. Vince-Prue, B. Thomas and K. E. Cockshull, Academic Press, Orlando, 1984)). In temperature-responsive plants, exposure to cold temperatures promotes flowering by a process known as vernalization. Vernalization affects meristems directly, perhaps by causing them to become competent to perceive flowering signals (Lang, Encyclopedia of Plant Physiology, 15(Part 1):1371–1536, (ed., W. Ruhland, Springer-Verlag, Berlin, 1965)). Other environmental cues that can affect flowering include light quality and nutritional status.
The developmental state of the plant can also influence flowering time. Most species go through a juvenile phase during which flowering is suppressed and then undergo a transition to an adult phase in which the plant becomes competent to flower (Poethig, Science, 250:923–930 (1990)). This “phase change” permits the plant to reach a proper size for productive flowering.
The influence of the development state of a plant on flowering timing is controlled along developmental flowering pathways. In the flowering literature, the developmental flowering pathways are often referred to as autonomous to indicate that they do not involve the sensing of environmental variables. However, it is unlikely that the autonomous and environmental pathways are entirely distinct. For example, day-neutral species of tobacco typically flower after producing a specific number of nodes and, thus, could be considered as flowering entirely through an autonomous pathway. However, grafting studies have indicated that both day-neutral and photoperiod-responsive tobacco species respond to similar translocatable flowering signals (Lang et al., Proc. Natl. Acad. Sci., USA, 74:2412–2416 (1977); McDaniel et al., Plant J., 9:55–61 (1996)). Accordingly, aspects of the underlying biochemistry of these pathways appear to be conserved.
Genetic analyses of several species has identified genes that affect the time in which a plant begins to flower. The most extensive genetic analysis of these genes has been performed in the plant species, Arabidopsis thaliana. 
In Arabidopsis, genes which control flowering timing have been identified by two approaches. One approach has been to induce mutations in early-flowering varieties so as to elicit either late-flowering or early-flowering. Late-flowering mutations identify genes whose wild-type role is to promote flowering, while early-flowering mutations identify genes that inhibit flowering. Studies in Arabidopsis have identified over 20 loci whose mutations specifically affect flowering time, and several other loci that affect flowering time as well as other aspects of development (e.g., det2, copl, gal and phyB) (Kooruneef et al., Ann. Rev. Plant Physiol., Plant Mol. Biol., 49:345–370 (1998); Weigel, Ann. Rev. Genetics, 29:19–39 (1995)).
A second approach to identifying flowering timing genes is to determine the genetic basis for the naturally occurring variations in flowering time. Although early-flowering Arabidopsis varieties are the most commonly used varieties in the lab, most Arabidopsis varieties are actually late-flowering. Late-flowering varieties differ from early-flowering varieties in that the late-flowering varieties contain dominant alelles at two loci, FRIGIDA (FRI) and FLOWERING LOCUS C (FLC), which suppress flowering (Sanda et al., Plant Physiol., 111:641–645 (1996); Lee et al., Plant Journal, 6:903–909 (1994); Clarke et al., Mol. Gen. Genet., 242:81–89 (1994); Koornneef et al., Plant Journal, 6:911–919 (1994)).
Physiological analyses of the flowering timing mutants and the naturally occurring flowering timing variations indicate that flowering is controlled in Arabidopsis by multiple pathways (Koornneef et al., Ann. Rev. Plant Physiol., Plant Mol. Biol., 49:345–370 (1998)). For example, plants containing one group of late-flowering mutants (fca, fpa, fve, fy, ld) and plants containing the late-flowering FLC and FRI alleles are delayed in flowering during inductive (long-day) conditions and more severely delayed during short-day conditions. Studies have shown that vernalization of these late-flowering lines can suppress the late-flowering phenotype. Another group of late-flowering mutants (co, fd, fe, fha, ft, fwa, gi) exhibit minimal or no difference in flowering time when grown in short days compared to long days. This group also shows little or no response to vernalization. Moreover, double mutants within a group do not flower later than either single-mutant parent, whereas double mutants containing a mutation in each group flower later than the single-mutant parents (Koornneef et al., Genetics, 148:885–92 (1998)). A separate autonomous pathway appears to control the age or, more specifically, the developmental stage at which plants are competent to flower. This pathway is referred to as autonomous because mutations in this pathway do not affect the plant's photoperiod response. Recent studies of these mutations have shown changes in these mutants, such as alterations of trichome patterns, which indicate that such mutant plants are delayed in transitioning from the juvenile to adult states (Telfer et al., Development, 124:645–654 (1997)). Accordingly, there appears to exist parallel flowering pathways which mediate flowering time in response to environmental and developmental cues.
The time in which plants flower is of great importance in both agricultural and horticultural crops. In horticultural crops, the product is often the flower, while in food, feed or fiber crops, the product is often the flower and/or the products of flowering (i.e., fruits, seeds, or seedpods). Understanding the molecular aspects of flowering timing control in these crops will lead to strategies for optimizing flower, fruit, and seed production. For example, accelerating the onset of flowering in certain crops may permit those crops to be grown in regions where the growing season is otherwise too short, or permit multiple crops to be grown in regions where only one crop is currently possible. In addition, preventing or substantially delaying flowering will increase the yield of the useful parts of certain crops. For example, delaying or preventing flowering in forage crops (e.g., alfalfa and clover) and vegetables crops (e.g., cabbage and related Brassicas, spinach, and lettuce) should increase crop yields. Likewise, the yields of crops in which underground parts are used (e.g., sugar beets or potatoes), may also be increased by delaying or preventing flowering. In sugar beets, the prevention of flowering will also permit more energy to be devoted to sugar production. Likewise the yield of wood and biomass crops may also be increased by delaying flowering.